An oscillometric (OSC) method and a K sound method are known as methods for measuring a body's blood pressure value.
The oscillometric method is a method of pressurizing an artery with a cuff, converting changes in the volume of the artery accompanying a heartbeat during a process of reducing the pressurizing pressure of the cuff (cuff pressure) into variations in pressure in the cuff (pulse wave amplitude), and calculating the blood pressure value by analyzing the variations.
Specifically, the pressure in the cuff is detected in the process of reducing the cuff pressure after the cuff pressure is increased to a value that is sufficiently greater than a systolic blood pressure value. Then, the cuff pressures and pulse waves superimposed thereon are extracted from the detected pressure in the cuff, and an envelope that stores the amplitudes of the pulse waves in a time series in pairs with the cuff pressures is created.
Based on the envelope, the cuff pressure at the time of generation of the pulse wave with the greatest amplitude is set as the average blood pressure value. Then, among the cuff pressures that are greater than the average blood pressure value, a cuff pressure at the time of generation of a pulse wave with an amplitude that is the closest in value to a predetermined percentage (e.g., about 50%) of the maximum value is set as the systolic blood pressure value. Also, among cuff pressures that are lower than the average blood pressure value, a cuff pressure at the time of generation of a pulse wave with an amplitude that is closest in value to a predetermined percentage (e.g., 60%) of the maximum value is set as the diastolic blood pressure value.
The K sound method is a method of detecting blood flow sounds (Korotkoff sounds=K sounds) of a measurement subject and determining the cuff pressure at the time when the K sounds start to occur as the “systolic blood pressure” and the cuff pressure at the time when the K sounds disappear as the “diastolic blood pressure”.
In the past, blood pressure measurement apparatuses that use both an OSC method and a K sound method have been proposed, and for example, Patent Document 1 discloses a blood pressure measurement apparatus that displays a blood pressure value measured using an OSC method and a K sound method.
Also, Patent Document 2 discloses a method of using a K sound method to measure a blood pressure value and using an OSC method to evaluate a blood pressure value obtained using the K sound method in order to determine whether or not the blood pressure value is correct.
Also, Patent Document 3 discloses a system in which an envelope of pulse wave amplitudes and an envelope of K sound amplitudes are created, overlaid on one another, and output by printing.